The present invention relates to a method of feedback-controlling the air-fuel ratio of an internal combustion engine, and more particularly, it relates to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the concentration of an exhaust gas ingredient.
Among conventional methods for feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (hereinafter referred to as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the concentration of an exhaust gas ingredient, the desired air-fuel ratio being set to a value leaner than a stoichiometric ratio, depending on operating conditions of the engine, there is a method proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 59-208141 wherein when the engine temperature is low (e.g. during warming-up of the engine), the desired air-fuel ratio is changed in an enriching direction according to the engine temperature.
In general, if so called lean-burn control in which the desired air-fuel ratio is controlled to a value leaner than the stoichiometric air-fuel ratio is performed when the engine temperature is low, misfire etc. is liable to occur due to an unstable combustion state of the air-fuel mixture, which results in degraded driveability. Therefore, according to the proposed method, the lean-burn control is inhibited when the engine temperature is low.
However, since this proposed method contemplates the engine temperature alone in determining whether the lean-burn control is to be carried out, there can be the possibility that the desired air-fuel ratio is set to the stoichiometric air-fuel ratio or a value richer than same even when the lean-burn control can be suitably carried out. Therefore, the proposed method is disadvantageous in respect of fuel consumption.